The present invention relates to an electric power storage system used for flattening the load of an electric power line and shifting the peak of the electric power line and, more particularly a sodium-sulfur battery system and a method of operating the sodium sulfur battery system and an electric power line system using the sodium-sulfur battery system.
The sodium-sulfur batteries using sodium for the negative electrode and sulfur the positive electrode have been received attention because of its large efficiency and its large energy density, and accordingly are expected to be used for an electric power storage system or for an electric vehicle. The sodium-sulfur batteries are contained in a thermal insulation container in order to maintain the operating temperature to 300 to 350xc2x0 C. to form a battery module, and the battery module is operated by being combined with an AC/DC converter unit which converts a direct current to an alternating current and also converts an alternating current to a direct current.
Such a battery module is usually used in order to flatten the load, and is repetitively and cyclically operated with charging electric power during nighttime and discharging the electric power at a constant rate during daytime.
Therefore, in taking into consideration the heat generation during discharging the electric power at the constant rate, the thermal insulation container has been designed so that the temperature of the battery does not exceed an allowable temperature. However, in a case where the battery modules are used for peak-shift in the electric power line system, the load pattern required by the electric power line differs depending on season, weather and so on. Therefore, in the case of discharging the electric power by coping with the pattern, there have been problems that the battery temperature exceeds the required upper limit temperature to deteriorate the reliability of the battery when the discharging current is large, and that the electric energy stored in the battery can not sufficiently used in the peak-shift when the discharging current is small. Particularly, there has been a problem that corrosion of the positive-polar container containing sulfur and sodium polysulfide is progressed to reduce the battery capacity and to reduce the efficiency when the battery temperature exceeds the required upper limit temperature.
In order to cope with the above-mentioned problems, Japanese Patent Application Laid-Open No.5-121092 and Japanese Patent Application Laid-Open No.8-17464 propose a method that the heat dissipating characteristic of the thermal insulation container is varied so as to keep the battery temperature within the required temperature range. However, this method has a new problem that the structure of the thermal insulation container becomes complex and the thermal insulation container becomes large in size and accordingly the reliability is deteriorated. If the thermal insulation container is designed based on a larger amount of heat dissipation by taking various kinds of discharge patterns into consideration, it is possible that the battery temperature does not exceed the required upper limit temperature during discharging. However, in that case, it is required to input a larger amount of energy into the heater provided in the thermal insulation container in order to keep the battery temperature within the required temperature range during charging and during standing-by, which causes a problem that the efficiency of the sodium-sulfur battery system is reduced.
An object of the present invention is to provide a sodium-sulfur battery system suitable for shifting the peak of an electric power line or a load, and a method of operating the sodium-sulfur battery system and an electric power line system using the sodium-sulfur battery system.
In order to attain the above object, the sodium-sulfur battery system in accordance with the present invention is composed of, at least, a battery module having a sodium-sulfur battery contained in a thermal insulation container; an AC/DC converter unit; a calculation means for calculating an amount of peak-shift of an electric power line or the load so that an amount of heat generation in battery and/or an amount of discharge of the battery module do not exceed an allowable amount of heat generation and/or an allowable amount of discharge of the battery module; and a control means for controlling discharge of the battery module based on the calculated result of the amount of peak-shift. It is preferable that the calculation means comprises a means for tentatively determining the amount of peak-shift; a means for calculating a value of discharge current of the battery module using a daily load characteristic of the electric power line or the load and a discharge characteristic of the battery module and the tentatively determined amount of peak-shift; a means for calculating an amount of heat generation in battery and/or an amount of discharge of the battery module; a means for comparing the calculated value of the amount of heat generation in battery with the allowable amount of heat generation in battery and/or the calculated value of the amount of discharge with the allowable amount of discharge; and a means for correcting the tentatively determined amount of peak-shift when the calculated values of the amount of heat generation in battery and/or the amount of discharge are larger than the allowable amount of heat generation in battery and/or the allowable amount of discharge, respectively, or when the calculated values of the amount of heat generation in battery and/or the amount of discharge are largely less than the allowable amount of heat generation in battery and/or the allowable amount of discharge, respectively. It is also preferable that the calculation means comprises a means for calculating the value of discharge current and the amount of discharge every time step during discharging the battery module and repeating the calculation until a discharge completion time determined by the tentatively determined amount of peak-shift elapses; and a means for feeding back the calculated value of the amount of discharge to the calculation of the value of discharge current. Further, the calculation means may comprise a means for functionally approximating the daily load characteristic of the electric power line or the load. Furthermore, the calculation means may comprise a means for capturing the heat dissipation characteristic of the battery module and/or temperature dependence of than the discharge characteristic of the battery module.
Further, in the sodium-sulfur battery system in accordance with the present invention, it is preferable that the calculation means comprises a means for calculating a value of discharge current of the battery module at each time point using the calculated result of the amount of peak-shift, and the control means comprises a means for controlling discharge of the battery module according to the amount of the discharge current, or that the calculation means comprises a means for determining a reference electric power level using the calculated result of the amount of peak-shift, and the control means comprises a means for controlling discharge of the battery module following to a load electric power exceeding the reference electric power level. Further, in the case of the latter, the sodium-sulfur battery system in accordance with the present invention may comprise a means for measuring the temperature of the sodium-sulfur battery or/and the amount of discharge of the battery module; a means for comparing the measured temperature or/and the measured amount of discharge with the allowable temperature of the sodium-sulfur battery or/and the allowable amount of discharge of the battery module, respectively; and a means for stopping discharge of the battery module when the measured temperature or/and the measured amount of discharge reaches the allowable temperature of the sodium-sulfur battery or/and the allowable amount of discharge, respectively, or when the measured temperature or/and the measured amount of discharge becomes larger than the allowable temperature of the sodium-sulfur battery or/and the allowable amount of discharge, respectively.
Further, a sodium-sulfur battery system in accordance with the present invention comprises a battery module having a sodium-sulfur battery contained in a thermal insulation container; an AC/DC converter unit; and a control means for controlling discharge of the battery module, wherein a maximum output power of the AC/DC converter unit is equal to or larger than an amount of peak-shift of the electric power line which is determined using a daily load characteristic of the electric power line or the load and a discharge characteristic of the battery module so that an amount of heat generation in battery or/and an amount of discharge of the battery module does not exceed the allowable amount of heat generation in battery or/and the allowable amount of discharge of the battery module.
The electric power line system in accordance with the present invention is an electric power line system using the sodium-sulfur battery system described above.
On the other hand, according to the present invention, in a method of operating a sodium-sulfur battery system comprising a battery module having a sodium-sulfur battery contained in a thermal insulation container, the method comprises the steps of calculating an amount of peak-shift of an electric power line or a load using a daily load characteristic of the electric power line or the load and a discharge characteristic of the battery module so that an amount of heat generation in battery and/or an amount of discharge of the battery module do not exceed an allowable amount of heat generation in battery and/or an allowable amount of discharge of the battery module, respectively; and controlling discharge of the battery module using the calculated result of the amount of peak-shift obtained in the calculation step. It is preferable that the calculation of the amount of peak-shift comprises the steps of tentatively determining the amount of peak-shift; calculating a value of discharge current of the battery module using the daily load characteristic of the electric power line or the load and the discharge characteristic of the battery module and the tentatively determined amount of peak-shift; calculating the amount of heat generation in battery and/or the amount of discharge of the battery module using the calculated value of discharge current; and correcting the tentatively determined amount of peak-shift when the calculated value of the amount of heat generation and/or the calculated value of the amount of discharge are larger than the allowable amount of heat generation and/or the allowable amount of discharge, respectively, or when the calculated value of the amount of heat generation and/or the calculated value of the amount of discharge are largely less than the allowable amount of heat generation and/or the allowable amount of discharge, respectively. Further, it is preferable that the calculation of the amount of peak-shift comprises the steps of calculating said discharge current and said amount of discharge every time step during discharging said battery module and repeating the calculation until a discharge completion time determined by the tentatively determined amount of peak-shift elapses; and feeding back the calculated value of the amount of discharge to calculation of the discharge current. Furthermore, the calculation of the amount of peak-shift may comprise a step of functionally approximating the daily load characteristic of the electric power line or the load. Furthermore, the calculation of the amount of peak-shift may comprise a step of capturing the heat dissipation characteristic of the battery module and/or temperature dependence of than the discharge characteristic of the battery module.
Further, the method of operating the sodium-sulfur battery system in accordance with the present invention is characterized by that a value of discharge current of the battery module at each time point is calculated using the calculated result of the amount of peak-shift, and discharge of the battery module is controlled according to the amount of the discharge current, or that a reference electric power level is determined using the calculated result of the amount of peak-shift, and discharge of the battery module is controlled following to a load electric power exceeding the reference electric power level. In the case of the latter, the temperature of the sodium-sulfur battery or/and the amount of discharge of the battery module is measured, the measured temperature or/and the measured amount of discharge is compared with the allowable temperature of the sodium-sulfur battery or/and the allowable amount of discharge of the battery module, respectively, and discharge of the battery module is stopped when the measured temperature or/and the measured amount of discharge reaches the allowable temperature of the sodium-sulfur battery or/and the allowable amount of discharge, respectively, or when the measured temperature or/and the measured amount of discharge becomes larger than the allowable temperature of the sodium-sulfur battery or/and the allowable amount of discharge, respectively.
According to the present invention, since the appropriate amount of peak-shift is calculated within the range of the allowable temperature of the sodium-sulfur battery or/and the allowable amount of discharge of the battery module and the discharge of the battery module is controlled based on the appropriate amount of peak-shift, the peak-shift of the electric power line or the load can be performed without deteriorating the reliability of the battery module and with effectively using the electric energy stored in the sodium-sulfur battery. Further, since there is no need to unnecessarily increase heat dissipation from the battery module, the sodium-sulfur battery system can be kept in a high efficiency without using a large amount of energy in order to keep the temperature of the battery during charging and standing-by. Further, since the maximum output of the DC/AC converter unit is appropriately selected, the peak-shift of the electric power line or the load can be effectively performed without using additional equipment. As the result, it is possible to effectively perform the peak-shift and to provide the high reliable and high efficient electric power line system.
The allowable amount of heat generation in battery and the allowable amount of discharge of the battery module differ depending on structural design and required lifetime of the battery module. The former is determined by an allowable temperature of the battery which is determined by a heat capacity, an amount of heat dissipation and a required lifetime of the battery module. For example, in order to secure a lifetime of 15 years required by an electric power line, it is necessary to keep the temperature of the sodium-sulfur battery below 370xc2x0 C., and accordingly the allowable amount of temperature rise becomes 50xc2x0 C. providing that the maximum temperature of the sodium-sulfur battery during standing-by is 320xc2x0 C. In a case where effect of heat dissipation is small, the allowable amount of heat generation in battery becomes the product of the allowable amount of temperature rise and the heat capacity of the battery module. On the other hand, an allowable amount of discharge is determined by an amount of active substances such as sodium and sulfur. Since an availability of the active substances in the structure of a general sodium-sulfur battery is 80 to 90%, the allowable amount of discharge can be determined from an amount in the unit of coulombs which is produced by electrochemical reaction of the amount of the active substance multiplied by the availability.